1. Field of the Invention
This invention relates to the field of electromagnetic contactors and more specifically to a method of minimizing the magnitude of current flowing in the coil of an electromagnetic contactor yet maintaining sufficient coil current to ensure positive holding even in the presence of induced flux effects.
2. Background Information
Electromagnetic contactors are used for controlling large amounts of electrical power supplied to electrical motors having operating currents in the range of a thousand or more amperes. Electromagnetic contactors may be configured as simple switches or as motor starters having integral current sensors and overload control circuits. In its simplest form, an electromagnetic contactor is a high-current switch having bridging contacts which are actuated with a spring-biased solenoid. The bridging contacts are typically configured with a fixed contact and a movable contact coupled to the spring-biased solenoid. The spring biasing of the contactor mechanism utilizes a kickout spring and a contact spring. The kickout spring maintains the bridging contacts in a normally open position. To close the contacts, current is injected into the contactor coil to actuate the solenoid mechanism and to move the bridging contacts to a closed position. Before the bridging contacts can move between the open and closed positions, the solenoid must generate enough force to overcome the force exerted by the kickout biasing spring. The moveable contacts then accelerate toward the fixed contacts until the contacts touch. The force applied to the contacts in the closed position is controlled by the contact spring and current flowing in the contactor coil which in turn controls the solenoid actuator. Once the contacts are touching, the current in the contactor coil is maintained at a level sufficient to maintain the bridging contacts in the closed position. The current flow in the coil is then interrupted to return the contactor to the open, rest position.
In the past, electromagnetic contactors were constructed as open systems, wherein the currents delivered to the contactor coil were in ranges calculated to provide acceptable performance. Most often, the currents used to drive the contactor coil were far in excess of the actual current required to provide an operating margin thought to anticipate all worst-case operating conditions. This type of contactor design results in several problems. Since excess current levels are used, the contactor operates in an inefficient mode, resulting in wasted energy and excess heat. Furthermore, since large currents are delivered to the contactor coil when accelerating the contactor contacts to a closed position, the contacts often accelerate to a high speed, resulting in contact bounce when the contacts ultimately close.
One system designed to overcome these problems is disclosed in U.S. Pat. No. 4,893,102, incorporated herein by reference. This system provides a bounceless contact closing operation by sensing the amount of current flowing in the contactor coil on a half-cycle basis. Specifically, the contactor coil voltage is sensed by a contactor control system and compared against a memory menu of stored delay angles. The delay angles are applied to the conduction interval of a gating device which is coupled in series with the contactor coil to control the current flowing therethrough. Depending upon the voltage sensed, the current flowing through the coil is varied on a half-cycle basis by the control system. The energy supplied to the closing contacts is such that only sufficient energy is imparted to allow the contacts to move to the closed position with a decreasing velocity which approaches zero as the contacts come into contact with each other. Once the contacts are closed, the coil current is regulated at a predetermined holding level by varying the conduction angle of the gating device coupled to the contactor coil.
While this system provides a vast improvement over contactor systems used in the past, it is susceptible to certain failure modes in multi-phase systems. Specifically, it is known that the large currents flowing in the contacts may induce unwanted flux in the contactor coil. In a multi-phase contactor system, depending on coil orientation relative to the contacts, and the direction of the current flowing in the respective contacts, the induced flux can result in the partial cancellation of currents flowing in the contactor coil. This reduction in net holding current may result in unwanted contact "drop-out" even when the applied coil holding current is well above the calculated current range for providing stable contactor operation.
From the foregoing, no system is known which provides all of the advantages of the system of U.S. Pat. No. 4,893,182, while also eliminating contact "drop-out" due to unwanted induced flux effects.